Numerous medical devices exist today, including but not limited to electrocardiographs (“ECGs”), electroencephalographs (“EEGs”), squid magnetometers, implantable pacemakers, implantable cardioverter-defibrillators (“ICDs”), neurostimulators, electrophysiology (“EP”) mapping and radio frequency (“RF”) ablation systems, and the like (hereafter generally “implantable medical devices” or “IMDs”. IMDs commonly employ one or more conductive leads that either receive or deliver voltage, current or other electromagnetic pulses from or to an organ or its surrounding tissue for diagnostic or therapeutic purposes. The leads include bare or insulated coiled wire forming one or more tightly wound solenoid-like structures along the shafts. These tightly wound coils facilitate torque transfer, prevent “buckling” and allow the conduction of electrical signals to and from the proximal (system) end to the distal (patient) end of the device. The lead may represent a catheter, an ICD lead, a neurostimulation lead, a pacemaker lead and the like. When exposed to electromagnetic fields, such as for example those present in magnetic resonance imaging (“MRI”) systems, these leads may sustain undesired currents and or voltages that interact with the surrounding blood and tissue, potentially resulting in unwanted tissue heating, nerve stimulation or other negative effects resulting in erroneous diagnosis or therapy delivery.
The catheter-type lead may incorporate conductive surfaces for the transfer of diagnostic and therapeutic electromagnetic signals as well as mechanical torque transfer. The catheter-type lead includes a distal tip electrode, which is commonly used to deliver energy to the target tissue and to receive electrical signals from the tissue it contacts. The catheter-type lead also includes proximal electrodes, which are typically used to receive electrical signals from the tissue they are contacting. This type of catheter structure is encountered in cardiac ablation and EP mapping catheters, for example. The electrical contact between the proximal end of the catheter and the electrodes is typically made via a bundle of individually insulated wires or conductors. An outer coil structure is typically used for torque transfer and is not in contact with the electrodes. The outer coil and the wires sometimes sustain currents when exposed to an electromagnetic field, such as for example that encountered in an MRI system. These currents may induce heating or cause nerve stimulation in the tissue surrounding the device either directly or by creating current pathways through the tissue that interacts with the electrodes.
Another example of a lead is a pacemaker or ICD lead which incorporates conductive wires for the transfer of diagnostic and therapeutic electromagnetic signals, as well as mechanical torque transfer. The lead includes a distal tip electrode, which is commonly used to deliver energy to the target tissue and to receive electrical signals from the tissue it contacts. The lead also includes a proximal electrode, which is mostly used to receive electrical signals from the tissue in its vicinity. In pacemaker and ICD leads, the conductive paths or coiled wires are connected to the electrodes, and are typically surrounded by dielectric materials. The conductive paths provided by coiled wires can sustain unwanted currents when exposed to an electromagnetic field, such as for example encountered in an MRI system. These currents can induce heating in the tissue surrounding the device either directly or by creating current pathways through the tissue involving the electrodes and the pathways.
One approach to form the braiding of a lead is to wind a bare, thin wire on a flexible former. In some constructions, a thin insulated wire is used instead of the bare wire in an attempt to form an inductor extending along the full length of the lead. The inductor acts as a “choke” to suppress currents from propagating along the body of the lead. Because of the small pitch utilized, the formed coil, even with wire insulated, may not be entirely electrically equivalent to a pure inductor over the full frequency spectrum of interest.
More recently, an alternative coil structure has been proposed that is referred to as a “zebra” coil. The zebra coil structure includes a series of insulated coil segments that are separated by non-insulated, bare coil segments. The bare segments of the coil conductor interconnect the insulated coil segments. The series of insulated coil segments form a series of self resonant RF chokes in the lead body and operates to reduce MRI RF heating. The RF chokes represent low pass filters, as in discrete inductors, and are generated by the inductance and capacitance in the insulated coil segments.
However, opportunities still remain to improve upon the performance of the existing zebra coil structure. In the existing zebra coil, it is preferable that the insulated segments are long enough to minimize the electromagnetic interactions or couplings between the insulated coil segments. However, as the bare coil segments increase in length the potential increases that the bare coil segments may introduce unfavorable high DC resistance in the lead body.
Also, it is preferable that the zebra coil exhibit stable self resonance such that the resonant frequency of each RF choke does not vary substantially. The resonance frequency of the RF chokes, created by the insulated coil segments, is impacted by the DC resistance of the non-insulated, bare coil segments. Thus, as the DC resistance of the bare coil segments varies up/down, the resonant frequency of the RF chokes varies.
During operation, once a lead is implanted, the lead body will be deformed cyclically, such as with heart beats. This means that adjacent turns in the bare coil segments may cyclically move between states in which adjacent turns transition between a state where they electrically engage with one another and electrically disengage from one another. Also, certain types of leads include a single filar or wire in each coil, while other types of leads include multiple filars in each coil. Hence, the potential exists that adjacent filars in a multi-filar coil will also move between engaged and disengaged states throughout the deformation cycle. The changes in conductive connections between adjacent filars and adjacent turns in the coil, present an unstable mechanical connection which causes the conductive pathway to continuously, cyclically vary. Hence, the DC resistance also varies continuously and cyclically in the bare segments which will impact the resonant frequency of the RF chokes created by the insulated coil segments.
A need remains for an improved MRI compatible lead that addresses the above problems and other issues that will be apparent from the following discussion and figures.